Daily and monthly output from seven evapotranspiration models (FAO-24 Radiation, FAO-24 Blaney-Criddle, Hargreaves-Samani, Priestly-Taylor, Makkink, and Turc) have been tested against reference evapotranspiration data computed by the FAO-56 Penman-Monteith model to assess the accuracy of each model in estimating grass-reference evapotranspiration in Louisiana. Models were compared at eight stations of the Louisiana Agriclimate Information System using data from December 2002 to November 2003. Comparisons were also made using three composite regions: statewide, inland, and coastal. A pan evaporation to reference evapotranspiration model (FAO-24 Pan Evaporation) was also tested against daily grass reference evapotranspiration from the FAO-56 Penman-Monteith model using data from two pan evaporation sites.

Statewide and in the coastal region the Turc model was the most accurate daily model with a mean absolute error (MAE) of 0.26 mm/day and 0.27 mm/day, respectively. Inland the FAO-24 Blaney-Criddle performed best with a MAE of 0.31 mm/day. On a monthly basis, Turc again performed the best statewide and in the coastal region (MAE 0.17 and 0.27 mm/day respectively). Inland, FAO-24 Blaney-Criddle and Makkink tied for the most accurate model, although this may change with a longer dataset. Pan evaporation at both stations performed poorly with MAE values over 1.0 mm/day. It is possible that the equation for calculating the pan coefficient that is suggested by the FAO in the FAO-56 manual may not be a suitable equation for use in Louisiana. These results will assist agricultural and environmental planners in assessing the available water resources in Louisiana.